Laser interferometric method for ns-time scale thermal mapping of Smart Power ESD protection devices during ESD stress

A backside heterodyne interferometric technique is presented to study therm al effects in smart-power electrostatic discharge (ESD) protection devices during the ESD stress. The temperature increase in the device active area c auses an increase in the silicon refractive index (thermo-optical effect) w hich is monitored by the time-resolved measurements of optical phase change s. Thermal dynamics and spatial temperature distribution in different types of npn transistor structures biased in the avalanche multiplication or sna pback regime are studied with nanosecond time and micrometer spatial resolu tion. The activity and inactivity of the bipolar transistor action is indic ated by the dominant signal arising from the emitter or base region, respec tively. Hot spots have been found at the edges of the structures and attrib uted to the current crowding effect in the emitter. (C) 1999 Elsevier Scien ce Ltd. All rights reserved.